irb(main):001:0> h = Hash.new {|hash, key| hash[key] = []}
=> {}
irb(main):002:0> h.has_key?(:a)
=> false
irb(main):003:0> h[:a]
=> []
irb(main):004:0> h.has_key?(:a)
=> true
irb(main):005:0> g = Hash.new([])
=> {}
irb(main):006:0> g.has_key?(:a)
=> false
irb(main):007:0> g[:a]
=> []
irb(main):008:0> g.has_key?(:a)
=> false
irb(main):009:0>
I think it needs to be said that Hash is an object - where behaviour and data are complected (possibly even on the instance level) - while Map is a data structure where the Map module provides the necessary functions to consistently manipulate that data structure.
So with Maps custom behaviour is either accomplished by injecting custom behaviour (under the control of the user of the data structure) functions like the Access.key/2 example or by defining new new manipulation functions as part of a new module - possibly creating a new data structure altogether that the standard Map becomes a part of.
